When performing microchip capillary electrophoresis (CE), sample introduction and injection can significantly affect CE performance. Currently, electrokinetic injection is used almost exclusively for microchip CE sample injection. In a particular type of electrokinetic injection known as “pinched” injection, various electrodes in a first mode apply voltages in four intersecting channels to drive sample through the intersection to a waste reservoir. In a second mode, the voltages induce injection of only a plug of sample occupying the small volume in the intersection towards a separation channel. The voltage applied to the separation channel is different in the first mode, wherein sample is diverted to waste, compared to the second mode, wherein the plug of sample is injected into the separation channel. Except for the sample plug, the vast majority of sample is wasted.
While electrokinetic injection can yield small sample plugs for improved separation efficiency and can minimize electrophoretic injection bias under certain conditions, it also has several significant limitations. For example, a considerable amount of time is required to achieve steady state in the first mode. Steady state is a necessary condition to avoid sample bias and/or injection bias caused by high mobility species arriving more quickly than low mobility species. During prolonged operation, the high mobility species can be depeleted preferentially and prematurely from the sample supply. Sample utilization is extremely inefficient because the total volume required is very large compared to the actual injected plug volume, which is very small. Furthermore, the injection volume is fixed because it is determined by the geometry of the intersection. In order to change the injection volume, the geometry of the intersection must typically be altered. Further still, the rate at which sequential injections can be analyzed, and the total number of sample plugs that can be injected into the separation channel, is limited by the steady-state flow requirements and by the changing voltages in the separation channel associated with electrokinetic injection.
In view of at least the limitations described above, a need for an improved sample injector and microchip CE system exists.